Multiple image transfer device

ABSTRACT

A device for transferring electrostatic toner images to a receiver is provided wherein a receiver is attached to a roller segment having a relatively large diameter. The roller segment is mounted in a carriage for movement along a path past a plurality of image transfer stations. A cycloidal cam surface controls rotational movement of the roller segment so that the toner images are transferred to the receiver substantially without smearing. The relatively large diameter of the roller segment assures a longer contact time at each station to assure more complete transfer than would result with a smaller radius transfer roller or roller segment.

[ Feb. 29, 1972 United States Patent Pollock [54] MULTIPLE IMAGE TRANSFER DEVICE 3,520,603 7/1970 Gnage.......m..............................355/4 [72] Inventor: John S. Pollock, Rochester, N.Y. Primary Examiner john M. Hora" [73] Assignee: Eastman Kodak Company, Rochester, A i an ExaminerTh0mas A. Mauro N.Y. Attorney-Robert W. Hampton and Gary D. Fields [22] Filed: Dec. 5, 1969 ABSTRACT [2H PP 882,644 A device for transferring electrostatic toner im ages to a receiver is provided wherein a receiver is attached to a roller segment having a relatively large diameter. The roiier segment is mounted in a carriage for movement along a path past a plu- 84 on 55 51 3g 3 mm 5" 3" rality of image transfer stations A cycloidal cam surface controls rotational movement of the roller segment so that the toner images are transferred to the receiver substantially Field of without smearing. The relatively large diameter of the roller segment assures a longer contact time at each station to assure [56] References Cited UNITED STATESPATENTS 2,986,466 5/1961 Kaprelian... 3,357,830 12/1967 more complete transfer than would result with a smaller radius transfer roller or roller segment.

14 Claims, 4 Drawing Figures Bixby............ 3,519,343 7/1970 PATENTEDFEBZS I972 sum 1 or 3 JOHN s. POLLOCK' INVENTOR.

ATTORNEYS PATENTEUFEBZB I912 SHEET 2 BF 3 JOHN 1 POLLOCK INVENTOR.

ATTORNEYS PAIENIEUFEBZSIWZ 3.645.617

JOHN s. POLLOCK INVENTOR.

BY A. 13344, $30M ATTORNEYS 1 MULTIPLE IMAGE TRANSFER DEVICE CROSS-REFERENCE TO RELATED APPLICATION Oliver W. Gnage, filed July l, 1968 and both entitled Image Transfer Mechanism, now US. Pat. Nos. 3,520,603 and 3,551,046 respectively.

BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention relates to transfer of an electrostatic toner image and more particularly to the transfer of a plurality of toner images in registry without smearing.

2. Description of the Prior Art In one form of electrophotography, a plurality of photoconductive members may be charged and imagewise exposed to color separation negatives to form electrostatic images thereon. These images may then be developed by different colored developers to form color toner images. Finally, the toner images may be transferred in registry to a receiver to form a color print. In commonly assigned US. Pat. Nos. 3,520,603 and 3,551,046, a receiver is wrapped around a roller which is moved along a linear path past a plurality of receiving stations wherein photoconductive members bearing toned electrostatic images are positioned for transfer by rolling contact of the receiver sequentially across each of the photoconductive members. In each instance, the. transfer roller is driven by a gear and rack arrangement so that the rotary speed of the receiver is controlled by the linear speed of the transfer mechanism. While these devices work satisfactorily, the contact time between the photoconductive surface and the receiver is of relatively short duration due to the relatively small diameter to the transfer roller. These configurations may result in incomplete image transfer. In US. Pat. Ser. No. 741,387 relative slippage could occur between receiver and photoconductive surface resulting in image smearing.

SUMMARY OF THE INVENTION In accordance with the present invention, an image transfer device is provided which reduces the possibility of smearing and increases the contact time between a photoconductive member bearing a toner image and a receiver to which the image is to be'transferred over the contact time provided by prior art devices. This is accomplished by utilizing a rotatable image transfer means having an arcuate surface in the form of at least a segment of a cylinder, the cylinder having a circumference which is greater than the pitch distance between two adjacent toner images at their respective transfer stations. Pitch distance as used throughout the specification and claims means the distance between corresponding points on adjacent photoconductive members when the photoconductive members are at their respective transfer positions. Means is provided for moving the image transfer means along a path past the transfer stations and cam means provides rotational movement of the arcuate surface across each of the toner images to effect transfer. The rotational movement of the arcuate surface is controlled by the cam means as the image transfer means is moved along the path.

More specifically, the transfer stations lie in a common plane. The receiver is attached to an arcuate surface of a roller segment having a large diameter. A rack and pinion arrangement imparts rotary motion to the roller segment upon translation of the roller along a path parallel to the common plane. However, this rotary motion during transfer is retarded by a cycloidal cam adjacent each transfer station. This is possible through a slip clutch arrangement between the gears which interconnect the roller segment and the rack and pinion. By increasing the diameter of the transfer roller the image transfer time is increased. Once an image has been transferred to the receiver the roller segment then rotates at a higher speed due to gear ratio of the gears which drive it through the rack and pinion so that it completes its circular motion in the time that the carriage is moved from the end of one transfer station to the beginning of the next transfer station so that the receiver will be in position to receive the next image. During the following cycle the movement of the roller segment is controlled by the cycloidal cam at that station as previously described.

Additional advantages of this invention will become apparent from the description which follows, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary perspective view of a multiple image transfer device constructed in accordance with this invention;

FIG. 2 is a diagrammatical section showing sequential positions of the image transfer means during movement from one transfer station to the next.

FIG. 3 is an offset vertical section, taken along line 3--3 of FIG. 2, showing a drive mechanism for the transfer device; and

FIG. 4 is a fragmentary section through a transfer station showing details of the transfer means and photoconductive member.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with this invention an image transfer device is provided for sequentially transferring toner images to a receiver in registry from separate photoconductive members positioned in a common plane at equal pitch distances. Thus, as shown in FIG. 1, a plurality of photoconductive members, such as photoconductive members P, P and P" are positioned in separate transfer stations. Each photoconductive member includes a photoconductive surface or layer 2 on a conductive backing 4, as best seen in FIG: 4. Each photoconductive member is moved into its respective transfer station from a development station, such as a liquid development station (not shown) by a transport 6 to which it is held, as by electromagnetic attraction. Each transport is driven, as by a timing belt 7 connected to a suitable power source, not shown. Each transfer station has registration pins 8, l0 and 12 respectively to space the photoconductive members at equal pitch distances D.

An image transfer device T is mounted upon a carriage C for linear movement past each transfer station. Carriage C includes a generally rectangular frame 14 mounted on spaced rails 16 and 18 by means of rollers 20 for movement along a path parallel to the common plane containing photoconductive members when they are in the transfer station. The carriage may be driven as by motor, not shown, connected thereto, as by timing belt 22.

Image transfer device T is generally U-shaped, having an arcuate transfer element surface 24 supported between spaced legs 26 and 28. Surface 24 may be coated with an elastomeric layer 30, such as conducting rubber, as best seen in FIGS. 1 and 4. A suitable receiver, such as paper 32, which may have a conductive backing, now shown, is attached to conductive layer 30 as by clamps, such as clamps 34 shown in FIG. 3. These clamps have been shown by way of illustration only. It will be understood that other attaching devices such as spring clamps or vacuum means can be provided.

Conveniently, arcuate surface 24 takes the'form of a segment of the surface of a right circular cylinder having a circumference substantially greater than pitch distance D. The arcuate surface 24 is mounted by means of legs 26 and 28 for rotation about a shaft 36, the shaft being joumaled in the walls of carriage frame 14 as shown. Shaft 36 is rotatably driven by a pinion 38 which in turn is driven by a rack 40 extending parallel to the path of carriage C upon movementof the carriage along the path. However, pinion 38 is mounted for rotation about shaft 36 on a sleeve 42 so that it does not directly drive shaft 36. A small gear 44 is mounted for rotation about a stub shaft 46 extending from the side wall of frame 14 and forms part of a friction clutch. Thus, one face of gear 44 is provided with a disc 48 made of high-friction material and is urged into contact with the face of a large gear 50 by means of coil spring 52, as seen in FIG. 3. By this means large gear 50 is driven by small gear 44 through friction disc 48. Gear 50 in turn drives a small gear 54 which is fixedly mounted on shaft 36 so that the shaft is turned thereby to rotate image transfer device T. The gear train thus described results in image transfer device T being driven at a higher rotational velocity than pinion 38 when the friction clutch is not slipping.

A cam member 56 is provided adjacent each transfer station and has a cycloidal cam surface 58 engageable by a cam follower, such as roller 60 attached to an arm 62 extending from leg 28, as shown.

J ust prior to image transfer, image transfer device T is in the position shown in solid lines at the left of FIG. 2 wherein the cam follower 60 is in engagement with the upper portion of cam surface 58 but arcuate surface 24 has not yet been brought into a position wherein a receiver attached thereto is in contact with photoconductive surface 2. As carriage 14 is moved to the right by timing belt 22, as viewed in FIG. 2, transfer device T will tend to be rotated in a counterclockwise direction by pinion 38 through the gear and clutch arrangement described above. However, the rotational movement of the image transfer device will be retarded by cam 56 so that a pure rotary motion of receiver 32 across photoconductive surface 2 is obtained so that any tendency to smear the transferred image is reduced. At the beginning of the cycle, a stop pin 64 which extends laterally from gear 54 is in engagement with a stop pin 66 extending from the hub of pinion 38, as best seen in FIGS. 2 and 3. However, the gear ratios are such that pinion 38 tends to rotate faster than image transfer device T is permitted to rotate due to the cam 56. Slippage of pinion 38 with respect to image transfer device T is permitted through the friction clutch.

As the carriage continues to move, cam follower 60- will move downwardly along cycloidal cam surface 58 so that the image transfer device reaches position T as shown in FIG. 2, wherein the leading edge of receiver 32 comes in contact with photoconductive surface 2.. Conveniently, photoconductive member P and transport means 6 are held against the upward force exerted by the image transfer device, as by spaced stops 68. Stops 68 may be solid or resilient.

In position T it will be noted that pinion gear 38 and hence stop pin 66 have moved ahead of stop pin 64 and small gear 54. Continued movement of the carriage to the right causes image transfer device to move through position T and finally to position T in substantially nonslipping rolling contact wherein the trailing edge of the image transfer device is brought into contact with photoconductive member 2. It can be seen that because of the large diameter of arcuate surface 24 the time of contact between photoconductive member P and receiver 32 will be relatively long so that a more complete image transfer may occur. This is true because the large diameter are of surface 24 causes a meniscus of developer liquid over a broad area so that the effective transfer time is increased. Also, elastomeric layer 30 is flattened across a larger area by a large diameter surface than it would be if surface 24 had a smaller radius. During the rolling movement of receiver 32 across photoconductive member 2, it will be noted that stop pins 66 and 64 move further and further apart due to relative slippage between gears 50 and 44 at friction disc 48.

Once cam follower 60 leaves cam surface 58 at position T however, the velocity of the image transfer device T greatly increases since the ratio of the gears is such that image transfer device T tends to be driven at a higher rotational speed than pinion 38. Because of this gear ratio, the image transfer device moves rapidly through positions T T T and T to position T at the next transfer station whereat roller 60 is in engagement with the next cam 56. Conveniently, a resilient bumper or stop 69 may be provided adjacent the upper end of surface 58 of earns 56 to absorb the energy of transfer device T when cam follower 60 strikes cam 56. It will be noted that by the time image transfer device T reaches position T stop pins 64 and 66 will again be in engagement, as shown. Should the pitch distance be sufficiently great that the stop pins come into contact prior to reaching position T they will cause friction clutch 48 to slip so that the relative positions of pinion 38 and image transfer device T remains the same until the cam follower 60 engages a cam surface 58 of a subsequent cam 56. Next, transfer deice T moves through position T to bring the leading edge of the image receiver 32 into contact with the next photoconductive member P.

To enhance transfer, a potential may be applied to image transfer device T having a polarity opposite the charge on the toner particles. This potential may be applied, as by a potential source 70 connected to cam 56. In some applications it is desirable to increase this potential with each successive transfer whereupon a higher potential 72 can be applied through cam 56 at the next transfer station. A still higher potential-74 can be applied at the next station, as seen in FIG. I.

From the foregoing, the advantages of this invention are readily apparent. The possibility of image smearing during transfer is reduced because rotation of the receiver sheet across the photoconductive surface is controlled by a cycloidal cam rather than by the drive mechanism. In addition, since the image receiving surface has a relatively large radius curvature, a longer effective transfer time is provided between the photoconductive surface and the receiver than would otherwise be possible to improve transfer of toner from the photoconductive surface to the receiver. It will be understood that additional stations could be provided for applying lacquer to a complete print. It can be seen that this device has application to making color prints comprising three or more color separation originals.

The invention has been described in considerable detail with reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Iclaim:

1. A device for transferring a plurality of toner images in registry, said device comprising:

means for positioning at least two toner images at separate transfer stations at a given pitch distance; image transfer means rotatable about a longitudinal axis and movable along a path past said transfer stations, said image transfer means having an arcuate surface in the form of at least a segment of a cylinder having a circumference which is greater than said pitch distance; means for moving said image transfer means along said path past said transfer stations and for rotating said image transfer means about said longitudinal axis during movement of said image transfer means along said path; and

cam means associated with each of said transfer stations engageable by said image transfer means for controlling rotational movement of said arcuate surface across each of said toner images to effect transfer thereof to said surface as said image transfer means is moved along said path.

2. A device as claimed in claim 1, wherein said cam means includes:

a cycloidal cam at each of said transfer stations; and

a cam follower on said image transfer means engageable with said camto restrict rotational velocity of said arcuate surface about said axis when said arcuate surface is in rolling engagement with said toner images.

3. A device as claimed in claim I, wherein said moving and rotating means includes:

a carriage, said image transfer means being rotatably mounted thereon; means for advancing said carriage to move said image transfer means along said path;

a rack mounted parallel to said path;

a pinion mounted for rotation by said rack when said carriage is advanced; and

a friction clutch interconnecting said pinion and said image transfer means for rotating said image transfer means as said carriage is moved along said path.

4. A device as claimed in claim 3, said device further including:

stop means for limiting relative rotation between said pinion and said image transfer means when said cam means is not controlling rotational movement.

5. A device as claimed in claim 3, wherein said means further includes:

gear means tending to rotate said image transfer means faster than said pinion, said cam means acting to retard said rotational velocity by means of relative motion at said friction clutch.

6. A device for transferring a plurality of toner images in registry onto a receiver at separate transfer stations, said device comprising:

means for positioning a plurality of photoconductive members having toner images thereon at separate transfer stations so that the toner images lie in a common plane and are spaced at equal pitch distances from each other;

a carriage movable along a path parallel to said plane;

means for moving said carriage along said path;

image transfer means rotatably mounted on said carriage about an axis extending transversely of said path and having an arcuate surface for holding the receiver, said surface being a segment of a cylinder about said axis having a circumference which is greater than said pitch distance;

means for rotating said image transfer means about said axis at a predetermined rotational velocity when said carriage is moved along said path;

a cycloidal cam associated with each station; and

a cam follower on said image transfer means engageable with each said cam as said receiver engages each respective photoconductive member to restrict said rotational velocity of said image transfer means as said receiver rolls across each photoconductive member when said carriage is moved along said path.

7. A device as claimed in claim 6, said rotating means includes:

a gear train having a gear ratio tending to rotate said image transfer means faster than it would normally rotate during contact of the receiver on said photoconductive chips; and

friction clutch means in said gear train to permit relative movement between said image transfer means and at least part of said gear train when said cam follower is in engagement with one of said cams.

8. A device as claimed in claim 7 wherein said rotating means includes:

a rack extending parallel to said path and positioned to be engageable by a gear in said gear train.

9. A device as claimed in claim 8 wherein said gear train includes:

a pinion coaxially mounted with respect to said image transfer means and engageable with said rack for rotation upon movement of said carriage along said path;

a first gear mounted for rotation by said pinion, said first gear being smaller than said pinion;

a second gear coaxially mounted with respect to said first gear and being rotatable by said first gear through said friction clutch, said second gear being larger than said first gear; and

a third gear mounted for rotation by said second gear for rotating said image transfer means, said third gear being smaller than said second gear.

10. A device as claimed in claim 9 wherein said gear train includes:

stop means to limit relative movement between said gears when said cam follower is not in engagement with one of said earns.

11. A device for transferring a plurality of toner images in registry onto a receiver, said device comprising:

a plurality of transfer stations;

means for positioning a plurality of photoconductive members having toner images respectively at said transfer stations so that the toner images lie in a common plane and are spaced at substantially equal pitch distances from each other;

a carriage movable along a path parallel to said plane and having a shaft rotatably mounted about an axis parallel to and extending transversely of said path;

means for moving said carriage along said path between said stations;

image transfer means mounted on said shaft for rotation about said axis, said image transfer means having an arcuate surface spaced from said shaft for supporting a receiver for contact with toner images on said photoconductive members;

means for rotating said image transfer means about said axis during movement of said shaft along said path between said stations;

a cam follower on said image transfer means;

cam means, located at each transfer station for engagement by said cam follower, for controlling the rotational velocity of said image transfer member during image transfer; and

clutch means for providing relative movement between said rotating means and said image transfer means during image transfer.

12. A device as claimed in claim 1 1, further including:

means for attaching said receiver to said surface of said image transfer means.

13. A device as claimed in claim 11, wherein the toner images have a polarity and further including:

means for applying a potential to said receiver of a polarity opposite the polarity of the toner images to attract the toner images to the receiver.

14. A device for transferring a plurality of toner images in registry onto a receiver, said device comprising:

a plurality of transfer stations;

means for positioning a plurality of photoconductive members having toner images respectively at said transfer stations so that the toner images lie in a common plane and are spaced at equal pitch distances from each other;

a carriage movable along a path parallel to said plane;

means for moving said carriage along said path;

image transfer means mounted on said carriage for rotation about an axis parallel to and extending transversely of said path and having an arcuate surface for supporting a receiver, said surface being a segment of a cylinder having a circumference which is substantially greater than said pitch distance;

first means, operative in response to movement of said carriage along said path, for rotating a receiver on said surface about said axis in a predetermined direction and successively into transfer contact with said photoconductive members, said first rotating means providing a predetermined rotational velocity in said predetermined direction for said arcuate surface during image transfer at said stations, said predetermined rotational velocity being coordinated with movement of said carriage by said moving means so that no slippage occurs between a receiver supported on said surface and said photoconductive members; and

second means for rotating said image transfer means in said predetermined direction faster than said predetermined rotational velocity while said surface is out of transfer contact with said photoconductive members to align said surface for transfer contact with the next successive photoconductive member during movement of said carriage through one pitch distance. 

1. A device for transferring a plurality of toner images in registry, said device comprising: means for positioning at least two toner images at separate transfer stations at a given pitch distance; image transfer means rotatable about a longitudinal axis and movable along a path past said transfer stations, said image transfer means having an arcuate surface in the form of at least a segment of a cylinder having a circumference which is greater than said pitch distance; means for moving said image transfer means along said path past said transfer stations and for rotating said image transfer means about said longitudinal axis during movement of said image transfer means along said path; and cam means associated with each of said transfer stations engageable by said image transfer means for controlling rotational movement of said arcuate surface across each of said toner images to effect transfer thereof to said surface as said image transfer means is moved along said path.
 2. A device as claimed in claim 1, wherein said cam means includes: a cycloidal cam at each of said transfer stations; and a cam follower on said image transfer means engageable with said cam to restrict rotational velocity of said arcuate surface about said axis when said arcuate surface is in rolling engagement with said toner images.
 3. A device as claimed in claim 1, wherein said moving and rotating means includes: a carriage, said image transfer means being rotatably mounted thereon; means for advancing said carriage to move said image transfer means along said path; a rack mounted parallel to said path; a pinion mounted for rotation by said rack when said carriage is advanced; and a friction clutch interconnecting said pinion and said image transfer means for rotating said image transfer means as said carriage is moved along said path.
 4. A device as claimed in claim 3, said device further including: stop means for limiting relative rotation between said pinion and said image transfer means when said cam means is not controlling rotational movement.
 5. A device as claimed in claim 3, wherein said means further includes: gear means tending to rotate said image transfer means faster than said pinion, said cam means acting to retard said rotational velocity by means of relative motion at said friction clutch.
 6. A device for transferring a plurality of toner images in registry onto a receiver at separate transfer stations, said device comprising: means for positioning a plurality of photoconductive members having toner images thereon at separate transfer stations so that the toner images lie in a common plane and are spaced at equal pitch distances from each other; a carriage movable along a path parallel to said plane; means for moving said carriage along said path; image transfer means rotatably mounted on said carriage about an axis extending transversely of said path and having an arcuate surface for holding the receiver, said surface being a segment of a cylinder about said axis having a circumference which is greater than said pitch distance; means for rotating said image transfer means about said axis at a predetermined rotational velocity when said carriage is moved along said path; a cycloidal cam associated with each station; and a cam follower on said image transfer means engageable with each said cam as said receiver engages each respective photoconductive member to restrict said rotational velocity of said image transfer means as said receiver rolls across each photoconductive member when said carriage is moved along said path.
 7. A device as claimed in claim 6, said rotating means includes: a gear train having a gear ratio tending to rotate said image transfer means faster than it would normally rotate during contact of the receiver on said photoconductive chips; and friction clutch means in said gear train to permit relative movement between said image transfer means and at least part of said gear train when said cam follower is in engagement with one of said cams.
 8. A device as claimed in claim 7 wherein said rotating means includes: a rack extending parallel to said path and positioned to be engageable by a gear in said gear train.
 9. A device as claimed in claim 8 wherein said gear train includes: a pinion coaxially mounted with respect to said image transfer means and engageable with said rack for rotation upon movement of said carriage along said path; a first gear mounted for rotation by said pinion, said first gear being smaller than said pinion; a second gear coaxially mounted with respect to said first gear and being rotatable by said first gear through said friction clutch, said second gear being larger than said first gear; and a third gear mounted for rotation by said second gear for rotating said image transfer means, said third gear being smaller than said second gear.
 10. A device as claimed in claim 9 wherein said gear train includes: stop means to limit relative movement between said gears when said cam follower is not in engagement with one of said cams.
 11. A device for transferring a plurality of toner images in registry onto a receiver, said device comprising: a plurality of transfer stations; means for positioning a plurality of photoconductive members having toner images respectively at said transfer stations so that the toner images lie in a common plane and are spaced at substantially equal pitch distances from each other; a carriage movable along a path parallel to said plane and having a shaft rotatably mounted about an axis parallel to and extending transversely of said path; means for moving said carriage along said path between said stations; image transfer means mounted on said shaft for rotation about said axis, said image transfer means having an arcuate surface spaced from said shaft for supporting a receiver for contact with toner images on said photoconductive members; means for rotating said image transfer means about said axis during movement of said shaft along said path between said stations; a cam follower on said image transfer means; cam means, located at each transfer station for engagement by said cam follower, for controlling the rotational velocity of said image transfer member during image transfer; and clutch means for providing relative movement between said rotating means and said image transfer means during image transfer.
 12. A device as claimed in claim 11, further including: means for attaching said receiver to said surface of said image transfer means.
 13. A device as claimed in claim 11, wherein the toner images have a polarity and further including: means for applying a potential to said receiver of a polarity opposite the polarity of the toner images to attract the toner images to the receiver.
 14. A device for transferring a plurality of toner images in registry onto a receiver, said device comprising: a plurality of transfer stations; means for positioning a plurality of photoconductive members having toner images respectively at said transfer stations so that the toner images lie in a common plane and are spaced at equal pitch distances from each other; a carriage movable along a path parallel to said plane; means for moving said carriage along said path; image transfer means mounted on said carriage for rotation about an axis parallel to and extending transversely of said path and having an arcuate surface for supporting a receiver, said surface being a segment of a cylinder having a circumference which is substantially greater than said pitch distance; first means, operative in response to movement of said carriage along said path, for rotating a receiver on said surface about said axis in a predetermined direction and successively into transfer contact with said photoconductive members, said first rotating means Providing a predetermined rotational velocity in said predetermined direction for said arcuate surface during image transfer at said stations, said predetermined rotational velocity being coordinated with movement of said carriage by said moving means so that no slippage occurs between a receiver supported on said surface and said photoconductive members; and second means for rotating said image transfer means in said predetermined direction faster than said predetermined rotational velocity while said surface is out of transfer contact with said photoconductive members to align said surface for transfer contact with the next successive photoconductive member during movement of said carriage through one pitch distance. 